Present-day advanced aircraft require auxiliary power units (APUs) as a supply of electrical, hydraulic, and pneumatic power to secondary power systems of the aircraft. Generally speaking, the APUs are gas turbine units and must be highly reliable. In addition, compactness is also required. Most suitably, the APUs are then based upon a single shaft, constant speed, gas turbine having a high specific speed, single stage centrifugal compressor, a reverse flow annular combustor, and a single stage radial or axial inflow turbine. Shaft power is utilized to drive electrical generators and/or pumps and compressor bleed air extracted from the system prior to combustion to provide pneumatic power. For high bleed air output, it is necessary to design the compressor to operate adjacent to its maximum flow point, that is, near a so-called "choke" condition. The extraction of increasing amounts of shaft power at constant speed and constant turbine inlet temperatures from the choke point incrementally displace the compressor operating point to lower flows and are eventually limited by encroachment upon the compressor surge line.
The assignee of the instant invention has in the past embraced the problems as defined herein before and defined a solution as described in now issued U.S. Pat. No. 4,981,018 ('018). This solution called for a compressor construction for the APU wherein a compressor hub and associated blades of the impeller formed thereby are surrounded by an annular shroud. Bleed passages in the shroud which are angled in the direction of flow, both axially and radially, have been found to improve efficiency.
While the invention of the '018 patent advanced the state of the art, low operating temperature environmental operating conditions conspired to detract from the turbomachine's overall efficiency especially during startup of the APU's compressor.
The shroud bleed air of the '018 patent, which has been warmed during compression is reingested by the compressor at the compressor inlet. This warmed bleed air appears to stratify toward a compressor outer shroud wall. This warmed stratified bleed air enters an outer flow path of the compressor's inducer section and causes performance penalties to the turbomachine.
In cold ambient air during startup, cold air is drawn into the compressor inlet and in so doing cools inlet support struts and related structure in such a manner as to cause the compressor front shroud structure to pull away from the compressor rotor blades which further deteriorates the turbomachine's performance.
The invention to be described more fully hereinafter solves the problems just enumerated in an exceptionally simple manner.